A conventional projection-type video display apparatus 10 is illustrated in FIG. 1. An array 12 of projection cathode-ray tubes 14, 16, and 18 provide red, green, and blue images respectively. The cathode-ray tubes are provided with respective lenses 15, 17, and 19. The projected images are reflected by a mirror 20 onto a projection screen 22. Additional mirrors can also be utilized, depending on the particular geometry of the optical paths. The green cathode-ray tube 16 projects the green image along an optical path 32, which in this example is oriented substantially orthogonal to screen 22. In other words, the centerline of the optical path is at right angles to the screen. The red and blue cathode-ray tubes have respective optical paths 34 and 36, which converge toward the first optical path 32 in a non-orthogonal orientation defining angles of incidence .alpha..
Screens for projection-type video display apparatus are generally manufactured by an extrusion process utilizing one or more patterned rollers to shape the surface of a thermoplastic sheet material. The configuration is generally an array of lenticular elements, also referred to as lenticules and lenslets. The lenticular elements may be formed on one or both sides of the same sheet material, or on one side only of different sheets, which can then be permanently combined as a laminated unit or otherwise mounted adjacent to one another so as to function as a laminated unit. In many designs, one of the surfaces of the screen is configured as a Fresnel lens to provide light diffusion.
The screen 22 of FIG. 1, however, comprises a three-dimensional hologram 26 disposed on a substrate 24. Such a screen was disclosed in Applicants' co-pending and commonly assigned U.S. patent application Ser. No. 08/777,887, filed on Dec. 31, 1996, and entitled PROJECTION TELEVISIONS WITH THREE DIMENSIONAL SCREENS, which application is herein incorporated by reference. The hologram 26 is a print of a master hologram substantially forming a diffraction pattern that manages the distribution of light energy from the three projectors 14, 16, 18, and can be made variable across the width and/or height of the screen. The screen receives images from the projectors on an entrance surface side 28 and displays the images on an exit surface side 30, with controlled light dispersion of all the displayed images. The three-dimensional hologram 26 has a thickness of not more than approximately 20 microns.
The substrate 24 is preferably a highly durable, transparent, water-repellent film, such as a polyethylene terephthalate resin film. One such film is available from E. I. du Pont de Nemours & Co. under the trademark Mylar.RTM.. The film substrate has a thickness in the range of about 1-10 mils, equivalent to about 0.001-0.01 inches or about 25.4-254 microns. A film having a thickness of about 7 mils has been found to provide adequate support for the three-dimensional hologram disposed thereon. The thickness of the film does not affect screen performance in general, or color shift performance in particular, and films of different thickness may be utilized.
The screen 22 may further comprise a light transmissive reinforcing member 38, for example, of an acrylic material, such as polymethylmethacrylate (PMMA). Polycarbonate materials can also be used. The reinforcing member 38 is presently a layer having a thickness in the range of approximately 2-4 mm. The screen 22 and the reinforcing member are adhered to one another throughout the mutual boundary 40 of the holographic layer 26 and the reinforcing member 38. Adhesive, radiation, and/or thermal bonding techniques may be utilized. The surface 42 of the reinforcing layer may also be treated, for example by one or more of the following: tinting, anti-glare coatings, and anti-scratch coatings.
Various surfaces of the screen and/or its constituent layers may be provided with other optical lenses or lenticular arrays to control particular performance characteristics of the projection screen, as is known to do with conventional projection screens. For example, the screen 22' may comprise a Fresnel lens 50 having a lens pattern 51 in physical contact with the entrance surface side 28 of the screen 22, as shown in FIG. 2. The lens pattern 51 has a plurality of horizontal ridges 52, which are parallel to one another along the width of the Fresnel lens 50, as shown in FIG. 3. As is well known to one having ordinary skill in the art, a lubricant is used between the Fresnel lens 50 and the substrate 24, and tape is used around the edges of the Fresnel lens 50 and the substrate 24 to fasten those two components together.
The embodiment of the screen 22 shown in FIG. 2 provides significantly improved performance over conventional screens: color shift performance is significantly improved; visibility is good over a larger range of horizontal viewing angles; a higher screen gain yields an increase in the overall brightness of the resulting image on the screen; the overall brightness is more uniform; and a higher resolution is possible. This performance, however, entails a significant manufacturing cost. For example, fastening the Fresnel lens 50 to the screen 22 to provide the screen 22' shown in FIG. 2 may contribute approximately 15% to 20% in material and labor costs to the cost of the screen 22'. This amount is significant, and is enough to render the screen 22' commercially impracticable.